In many areas of the world, fine quality, beautifully colored, transparent spinels are found and used as gems. The only gem-quality spinel reported thus far in Texas is black and opaque. Near Eagle Flat in Hudspeth County, black spinel crystals have been found associated with augite and natural glass; these minerals are weathering out of an intrusive igneous rock. The spinel crystals have an octahedral form which is common for this mineral (fig. 17). Most of the spinels are free of flaws, but because of their black color they have little value as gems. The crystals are found loose in the sand of streams near the outcrops of the igneous rock or embedded in the rock. They seldom exceed half an inch in diameter. These stones are primarily sought by collectors. [Illustration: Fig. 17. Common crystal form of spinel.] Tektite (Bediasite) _Composition_: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4% FeO, also MgO, Na₂O, K₂O, and traces of other elements. _Crystal structure_: amorphous. _Hardness_: 5 to 6. _Specific gravity_: 2.33 to 2.44. _Luster_: vitreous, often dull on weathered surfaces. _Color_: dark brown, greenish brown, appears black in thick sections. _Streak_: uncolored. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle. _Diaphaneity_: transparent to subtransparent. _Refractive index_: 1.488 to 1.512. The average bediasite size is about 1 inch in diameter, although specimens approximately 3 inches in diameter are known. The uncut tektites are very interesting, showing a variety of shapes and surface features (Pl. V, A) and many exhibit contorted flow structure. The surface of many tektites is grooved or furrowed, while on others it is smooth or frosted. The Texas tektites are known as “bediasites,” after place names in Grimes County traceable to the Bedias Indians who formerly lived there. Dark brown and greenish-brown tektites have been found in Texas in gravels at scattered localities in Walker, Grimes, Brazos, Burleson, Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas the only other authenticated tektite localities in the United States at the present time are in Dodge and Irwin counties, Georgia. A fragment of a similar tektite has recently been reported from near Martha’s Vineyard, Massachusetts. The tektites reported from Oklahoma are now known to be pebbles of obsidian. Although tektites have little value or beauty as gemstones, they have been cut by lapidaries as both faceted and cabochon stones. Tektites take a high polish but are mostly so dark in color that they appear black.